Improvements in the yield of cereal crops such as wheat and maize have recently plateaued (Grassini, P., Eskridge, K. M. & Cassman, K. G. Distinguishing between yield advances and yield plateaus in historical crop production trends. Nat. Commun. 4, 2918 (2013)). This is a major cause for concern with respect to food security, particularly given projected global population growth and the impacts of climate change on crop productivity. In intensive agricultural systems, intensive fertilization and irrigation cause environmental degradation and are not sustainable in the long term, while in the low-input agriculture characteristic of developing nations, limited availability of water and nutrients are primary limitations to crop production (Sutton, M. A. et al. Too much of a good thing. Nature 472, 159-161 (2011), Lynch, J. P. Roots of the second green revolution. Aust. J. Bot. 55, 493-512 (2007)). Therefore, crops and crop varieties with greater resource efficiency and climate resilience are urgently needed in global agriculture.
Anatomical traits have the potential to deliver major improvements in crop production, by improving resource capture, transport, and utilization (Lynch, J. P. Root phenes that reduce the metabolic costs of soil exploration: opportunities for 21st century agriculture. Plant. Cell Environ. 1775-1784 (2014), Postma, J. A. & Lynch, J. P. Root cortical aerenchyma enhances growth of Zea mays L. on soils with suboptimal availability of nitrogen, phosphorus and potassium. Plant Physiol. 156, 1190-1201 (2011)). For example, smaller diameter of xylem vessels improved water use efficiency, conserving water resources for grain filling in wheat (Richards & Passioura. A breeding program to reduce the diameter of the major xylem vessel in the seminal roots of wheat and its effect on grain yield in rain-fed environments. Aust. J. Agric. Res. 40, 943-950 (1989)). Reduced cortical cell file number and increased cortical cell size reduce root respiration and increase rooting depth, leading to improved water acquisition and greater yield under drought (Chimungu, J. G., Brown, K. M. & Lynch, J. P. Reduced root cortical cell file number improves drought tolerance in maize. Plant Physiol. 166, 1943-1955 (2014), Chimungu, J. G., Brown, K. M. & Lynch, J. P. Large root cortical cell size improves drought tolerance in maize. Plant Physiol. 166, 2166-2178 (2014)). The formation of root cortical aerenchyma (RCA), which converts living cortical cells to air space via programmed cell death, also improves crop growth and productivity under drought and suboptimal nitrogen conditions (Saengwilai, P., Nord, E., Chimungu, J., Brown, K. & Lynch, J. Root cortical aerenchyma enhances nitrogen acquisition from low nitrogen soils in maize (Zea mays L.). Plant Physiol. 166, 726-735 (2014), Chimungu, J. G. et al. Utility of root cortical aerenchyma under water limited conditions in tropical maize (Zea mays L.). Field Crops Res 171, 86-98 (2015)). Root anatomical traits, therefore, represent promising targets for crop breeding.
Despite this knowledge, anatomical traits have received little attention as selection criteria in plant breeding because of the challenges of sampling root systems from soil and quantifying anatomical phenotypes. Instead, genetic and physiological studies of root anatomical traits have been limited to artificial growth conditions, young plants, and few replications due to difficulties in obtaining and analyzing root cross-sectional images in a large number of samples. However, the recent development of high throughput phenotyping image analysis software, RootScan, permits quantitative measurements of anatomical traits from root cross-sectional images (Burton, A. L., Williams, M., Lynch, J. P. & Brown, K. M. RootScan: Software for high-throughput analysis of root anatomical traits. Plant Soil 357, 189-203 (2012)).
In view of the current state of the crop breeding industry, particularly new maize varieties, it can be appreciated that identifying genes conveying abiotic stress tolerance is a substantial challenge in the field. Accordingly, a need exists in the field to identify additional genes that influence stress tolerance.